1. Field of the Invention
The present invention relates to interference cancellation systems and methods, and more particularly relates to interference cancellation systems and methods for minimizing or eliminating interference in radio receivers due to unwanted signals. Even more specifically, this invention relates to an interference cancellation system and method for minimizing an interfering signal which arrives at different times at the radio receiver through multi-paths due to reflections or the like.
2. Description of the Prior Art
An adaptive interference cancellation system cancels interferences in a receive line by employing synchronous detectors, such as vector demodulators, signal controllers, such as vector modulators, and integrators, as the central elements of a closed control loop within the cancellation system that require no a priori knowledge of the interference signal characteristics, leaving the desired signal almost unaffected at the receive line.
FIG. 1 of the drawings illustrates a conventional adaptive interference cancellation system connected to a radio receiver system. The radio receiver system basically includes a receiver antenna 2, a receiver 4 and a receiver transmission line 6 interconnecting the receiver antenna 2 and the receiver 4. The receiver antenna 2 may be viewed as receiving both an interfering signal and a desired signal.
The interference cancellation system is designed to cancel the interfering signal from the receiver path defined by the receiver antenna 2, the receiver 4 and the receiver transmission line 6. The interference cancellation system accepts an RF sample of the interfering signal with the help of an auxiliary or reference antenna 8. This reference signal is used to detect the presence, amplitude and phase of this same signal in the receiver path or transmission line 6 between the receiver antenna 2 and the receiver 4.
A directional coupler 10 is electrically coupled to the receiver transmission line 6 to “tap” the receiver transmission line and provide a sample signal (also known as residual error sample). A portion of the reference signal is provided to one input port of a synchronous detector 12 using a directional coupler 14 which is electrically coupled to the auxiliary antenna 8. The other input of the synchronous detector 12 is provided with the sample signal from the directional coupler 10 of the receiver path.
The synchronous detector 12 compares the reference signal with the sample signal, and provides detector output signals which vary in accordance with the differences in amplitude and phase between the reference signal and the sample signal. The synchronous detector is generally a quadrature phase detector having two outputs, Q and I.
Each of the detector output signals may be provided to an integrator/amplifier 16, which will provide time varying, DC control signals which vary in response to the detector output signals. These control signals are provided to a signal controller 18.
The signal controller 18 receives the reference signal through an output of the directional coupler 14 and adjusts the amplitude and phase of the reference signal in response to the control signals it receives from the synchronous detector 12 (via the integrator/amplifier 16). An amplifier 20 may be positioned between the directional coupler 14 and the signal controller 18 to amplify that portion of the reference signal which passes through the directional coupler.
The signal controller 18 provides a cancellation signal which is injected into the receiver path using a subtractor 22 with equal amplitude but in a phase opposite to that of the interfering signal, thereby cancelling the interfering signal from the receiver path.
A problem with the conventional adaptive interference cancellation system shown in FIG. 1 is when the interference signal follows multiple paths, which can occur due to multiple reflections or the like. The interference signal is received by the radio receiver after multiple time delays and generally in different strengths due to the multiple paths that it follows. In such a case, the depth of cancellation of interference achieved in the conventional adaptive interference cancellation system can be greatly degraded, since no single delay line cancellation system can approximate multi-path effects.
Improved depth of cancellation of interference, arriving from its source to a receiving antenna through several propagation multi-paths, can be achieved by employing a multiple interference cancellation system, which employs a number of adaptive cancellers, the number being kept at the optimum by approximating two or more propagation paths to their single equivalent paths. The optimum number of approximated paths and consequently the number of adaptive cancellers are determined by the time spread over which the multi-path interference occurs, and the tap spacing (delay) that is required to achieve a desired depth of cancellation.
FIG. 2 is a block diagram which shows an arrangement of a conventional multi-path interference cancellation system 30 that employs several adaptive cancellers 32 with different time delays (effected by delay lines 34) at the reference paths of the cancellers. Letter “n” in the subscripts of the block diagram shown in FIG. 2 corresponds to the optimum number of approximated propagation paths, which is, conversely, consistent with the number of cancellers 32 in the system. The interference signal emanating from a source 33 takes multiple, time delayed paths (“1”, “2”, . . . “n−1”, “n”) 35 to the receive antenna 54 of the radio receiver system.
As shown in FIG. 2, the conventional adaptive multi-path interference cancellation system 30 includes a plurality of adaptive cancellers 32, each having a synchronous detector 36, integrators 38 having a fixed gain and bandwidth coupled to the outputs of the synchronous detectors 36, and a signal controller 40 coupled to the output of the integrators 38 as part of closed control loops each having a fixed gain and bandwidth. The interference signal which is received by the reference antenna 42 is divided, and the divided signals are provided to the various reference signal paths of the adaptive cancellers 32. Alternatively, and as shown in FIG. 2, directional couplers 44 or dividers 46 may be used which are electrically coupled to a transmission line 43 electrically coupled to the reference antenna 42 to provide a reference signal. The reference signals, either from the divider 46 or the directional couplers 44, are provided to the signal controller 40 of each adaptive canceller 32 through an adjustable delay line 34. Each delay line 34 is optimized to adjust for a corresponding delay in a single equivalent propagation path which approximates two or more propagation paths which the interference signal takes.
An error sample signal is extracted from the receiver transmission line 52 between a subtractor 50 and the radio receiver 56 using a directional coupler 58, signal splitter or the like. The error sample signal on the output of the directional coupler 58 is provided to the input of a splitter or divider 60, on whose outputs are provided multiple versions (“Error_1[A(T1)]”, “Error_2 [A(T2)]”, . . . “Error_n−1 [A(Tn−1)]”, “Error_n [A(Tn)]” of the error sample signal. These multiple error sample signals are provided to inputs of the synchronous detectors 36 of the adaptive cancellers 32.
The signal controller 40 of each adaptive canceller 32 provides a cancellation signal (“Cancelling_1”, “Cancelling_2” . . . , “Cancelling_n−1”, “Cancelling_n”), which cancellation signals are summed together using a combiner 48 to provide a combined cancellation signal coherent with the approximated characteristics of the multi-path interference (“Synthesized Cancelling Waveform”). This combined cancellation signal is provided to a signal subtractor 50, such as a 180° hybrid, to inject the combined cancellation signal into the received signal path (i.e., the transmission line 52 connected to the receive antenna 54) of the radio receiver system to cancel or at least minimize the multiple interference signal components of the received signal received by the receive antenna 54, leaving the desired signal received by the radio receiver 56 substantially free of the unwanted multi-path interference signal.
While the conventional adaptive multi-path interference cancellation system 30 shown in FIG. 2 adaptively accommodates attenuation and time delay, the same as those of the interference propagation paths, it does not take advantage of the time-varying trends of multi-path interferences, as will be explained in greater detail.